Information is written onto a magnetic tape by magnetizing tape elements. These magnetized tape elements produce a magnetic field which can be detected and converted to an electrical signal by a read head. A common type of read head for carrying out this conversion is the magnetoresistive (MR) read head.
A simple MR head consists of a thin film of magnetoresistive material, such as Permalloy, between two insulating layers. When the MR layer is formed, a magnetic field is typically applied in a direction parallel to the plane of the thin layer. Thus, the MR layer exhibits a uniaxial anisotropy with an easy-axis of magnetization parallel to the direction of the applied field. If an external magnetic field, such as from a magnetic tape, is applied normal to the easy-axis, the magnetization direction of the MR layer will rotate away from the easy-axis and towards the direction of the applied magnetic field. This magnetization rotation causes a change in resistance in the MR layer. When no external field is applied, the resistance is greatest. The resistance decreases with increasing applied field. For practical geometries of the MR layer, resistance as a function of applied field traces a bell-shaped curve. The MR head is often biased with an applied current such that a zero magnitude applied field results in a resistance near an inflection point on the resistance curve. Thus, small changes about a zero magnitude applied external field result in nearly linear changes in resistance.
To accommodate increasing densities of data stored on magnetic tape, the geometries of read heads continue to shrink. One difficulty encountered is the increasing affect of Barkhausen noise. As the width of the MR layer is narrowed, the MR layer tends to split into magnetic domains, resulting in demagnetization. In the presence of an increasing externally applied field, the domain walls make sudden movements, causing jumps in the output signal. Two methods exist to reduce or eliminate Barkhausen noise. The first is to increase the effective length of the MR layer. Lengthening the MR reduces demagnetization at the ends and, hence, results in a greater retention of a single magnetic domain. The main difficulty with this technique is that the resulting increase in read head size is contrary to the need for increased data density on magnetic tapes. The second technique uses a small magnetic field in the direction of the easy-axis to induce a single domain in the MR layer. An implementation of this method uses permanent magnetics placed over the ends of the MR layer. These magnets strongly pin the domains of the MR layer under the magnets and create a weak longitudinal magnetic field in the MR layer between the covered ends. Difficulties with this implementation include complex geometries and additional processing steps required to implement the additional permanent magnetic.
In addition to Barkhausen noise, cross-sensitivities to other parameters, such as temperature, can affect the performance of the MR head. A dual active element MR read head minimizes cross-sensitivities. The dual active element MR head includes two MR layers in parallel separated by an insulating layer. Two additional insulating layers, one on each end of the structure, insulate the MR layers from surrounding materials. The two MR layers are connected in parallel to a source current such that current flows in the same direction through both layers. The fringe field produced by current flowing through each MR layer biases the adjacent layer. Hence, an externally applied magnetic field produces an increase in resistance of one MR layer and a corresponding decrease in resistance of the other MR layer. A differential amplifier with an input connected to each MR layer converts these changes in resistance to an output voltage. Environmental changes to both MR layers, such as changes in temperature, appear as common mode inputs to the differential amplifier and, hence, are rejected.
What is needed is a dual active element MR read head with reduced Barkhausen noise susceptibility. The read head should be easy to produce, have simple geometry, and not require additional processing steps over prior dual active element MR read heads.